The client.cf file is now roughed out enough that it
can almost be used to send mail. All that remains are two important
loose ends: options, which we cover in this chapter, and headers,
which we cover in the next.

Options determine much of the behavior of sendmail.
For example, they can specify the location of the queue directory,
set the default permissions for files,
and list the characters that separate tokens in an address.
These and other options will be discussed in this chapter.

Options are declared in the configuration file by beginning a line
with the letter O:

OQ/var/spool/mqueue single-character name (prior to V8.7)

Prior to V8.7 sendmail, option names could only be a single character.
The short (single character) name of the option
(here Q) immediately follows
the O with no intervening
space.
The value assigned to a single-character option
immediately follows the option letter with no intervening space.

O QueueDirectory=/var/spool/mqueue multicharacter name (beginning with V8.7)exactly one space

To use multicharacter names, you must separate the name (here QueueDirectory)
from the O command with exactly one space character.
[1]
The value assigned to the multicharacter option
follows an equal sign.
The equal sign can be surrounded by optional spaces.

[1] If the short option name is a space, sendmail
presumes that the option name will be multicharacter.

Some options have both a single-character name and a multicharacter name,
in which case the two names are equivalent:

OQ/var/spool/mqueue define location of queue directory
O QueueDirectory=/var/spool/mqueue the same

Other options (the newer
ones) only have multicharacter names:

O ServiceSwitchFile=/etc/service.switch only multicharacter form available

When we mention such options in the text, we will specify the multicharacter name first,
followed by the single-character name in parentheses if there is one; for example, the
QueueDirectory (Q) option, but the ServiceSwitchFile option.

The values for
some options are strings (such as /tmp), while the values
for others can be numbers (such as 3), time durations (such as 3d
for three days), or a Boolean value (such as True). There are
no hard rules for which type of value goes with which option.
Instead, you'll need to look up each option in
Chapter 34, Options,
and use the type indicated there.